D. F. PEPPARD, J. P. FARIS, P. R. GRAYAND G. W. MASON
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STUDIES OF THE SOLVENT EXTRACTION BEHAVIOR OF THE TRANSITION ELEMENTS. I. ORDER AND DEGREE OF FRACTIONATION OF THE TRIVALENT RARE EARTHS BY D. F. PEPPARD, J. P. FARIS, P. R. GRAYAND G. W. MASON Contribution from the Chemistry Division, Argonne National Laboratory Received April 9, 1966
The extractability of the lanthanides into tributyl phosphate from an aqueous hydrochloric acid phase and from an aqueous phase 8 to 15.6 M in nitric acid increases with increasing atomic number, ,paralleling the theoretiaal order of decreasing basicity. This order is inverted for the system TBP and a 0.3 M nitric acid aqueous phase. Yttrium falls in the position predicted from consideration of ionic radii. The logarithms of the distribution ratios of a given pair of lanthanides diverge with increasing acidity for relatively concentrated nitric acid solutions, so that the theoretical maximum mutual separation of the pair requires the use of concentrated nitric acid. Use of an inert diluent for the TBP decreases the extractability of the rare earths by a factor much larger than that predicted on the assumption of simple dilution. The lanthanides may be divided into two groups, the split occurring at any predetermined point, by control of certain variables in countercurrent fractionation. In this way a valuable thulium-lutecium fraction has been obtained from a mixed rare earth source. Consequently, by means of two successive divisions any individual lanthanide may, in principle, be isolated from its neighbors on both sides. The lanthanides, yttrium and scandium may be purified with respect to many common impurities by extraction from a salted phase. Scandium may be separated from the lanthanides and yttrium by extraction from hydrochloric acid.
Introduction Studies of the fractionation of lanthanides by a liquid-liquid extractive technique have extended over the past fifteen-year period, the first work reported being that of Fischer, Dietz and Jubermann.’ The reviews by Bock,2 Quill3 and Wylie4 stress the importance of a continuous liquid-liquid extractive technique applicable to the fractionation of lanthanides. The present paper describes the study of tri-n-butyl orthophosphate, (n-C4HBO) oPO, hereinafter referred to as TBP, as the organic phase using a solution of trivalent rare earth chlorides or nitrates as the aqueous phase, and reports the development of chemical techniques held applicable to the continuous fractionation of rare earths by means of countercurrent liquid-liquid extraction. It is felt that the novelty of the TBP-nitric acid system, in comparison with previously reported systems, lies in the combinatioo of the large distribution ratio for a given lanthanide under properly chosen conditions and the relatively sharp differentiation between adjacent lanthanides. Experimental General.-TBP was washed with two one-fifth volumes of aqueous 5% sodium carbonate before use in any of the experiments described. (The purpose of the sodium carbonate washes is to ensure the absence of trace quantities of phosphoric acid, monobutyl phosphoric acid and dibutyl phosphoric acid.) I n all experiments, washed T B P was pre-equilibrated with respect to an aqueous phase identical to that under study except that it contained none of the element whose extraction was being investigated. In radioactive tracer studies, a quantity of tracer was added such that each milliliter of the feed solution corresponded to approximately 106 counts per minute under the counting conditions employed. Equilibration periods of five minutes were found t o be adequate. For radioactive assay, aliquots of the liquid were evaporat,ed on a five-mil platinum disc. The number of counts per minute associated with such an aliquot was then determined by the usual means.6 -
(1) W.Fischer, W. Dietz and 0. Jubermann, Naturwissenschaften, $8, 348 (1937).
(2) R. Bock, Aneew. Chem., 68, 375 (1950). (3) L. L. Quill, Record Chem. Progress, 11, 151 (1950). (4) A. W. Wylie, Roy. Australian Chem. Inst. J . Proe., 17, 377 (1950). (5) Beta counting was done on the first shelf of a Geiger-Mliller counter filled with neon-amyl acetate $88 with a window thickness of approximately 2 mg./cm.*, using an aluminum absorber (12.5 mg./ cm.9) to cut out stray alpha particles,
I n the determination of K , the distribution ratio (concentration in the organic phase divided by the concentration in the aqueous phase at equilibrium), for tracer elements two sets of measurements were made for which the quantity of tracer used in one set differed from that used in the other by a factor of 25 or more. In the study of lanthanide mixtures two methods of operation were employed. I n the technique entitled “countercurrent distribution” by Craig6 each separate portion of phase I remains in its given contactor in which it is contacted successively with separate portions of phase 11. In the method of operation referred to as “pseudo countercurrent extraction” by Hunter and Nash’ and discussed by them in some detail the phases are moved, batchwise, countercurrently, so that the batch analog of continuous column extraction results. I n the present work the pseudo-countercurrent extraction system was operated with both an extraction and a scrubbing section, the effluent aqueous scrub joining the influent aqueous feed and thus effecting reflux. Spectrographic Assays.-The rare earth content of each organic phase was extracted into water. Each water reextract, as well as each aqueous phase to be assayed, was washed with benzene or some similar solvent to remove traces of TBP. Aliquots were then converted to chlorides by repeated evaporation with hydrochloric acid. The resulting chloride solutions were assayed for rare earth content by the copper spark technique of Fred, Nachtrieb and Tomkins .E Other Assays.-Total rare earth content was determined by the usual procedure of precipitation of an oxalate, from an aqueous solution, followed by ignition of the oxalate to an oxide. Prior to the oxalate precipitation the aqueous phases, or aqueous re-extracts of organic phases, were scrubbed as above to remove traces of T B P and evaporated to remove excess acid. From the total rare earth content and the relative values obtained by spectrographic assay the content of specific rare earths was calculated. Acid content of aqueous phases (containing no salt) was determined by titration of a suitably diluted aliquot with aqueous sodium hydroxide, appkoximately 0.2 M , using phenolphthalein as indicator. Acid content of organic phases (containing no salt) was determined by the same technique except that the aliquot was diluted with absolute ethyl alcohol and the standard base was an ethyl alcohol solution. Acid content of feed (an aqueous phase containing salt) was determined in an approximate manner by electrometric titration of a diluted aliquot. Sources of Materials .-TBP and Gulf Solvent B T were obtained from Commercial Solvents Corporation and Gulf Refining Company, respectively. The rare earths were ob(6) L. C. Craig, Anal. Chem., 21, 85 (1949). (7) T. G. Hunter and A. W. Nash, Ind. Eng. Chem., a7, 836
(1935). (8) M.Fred, N. H. Nacrhtrieb and F. 8. Tomkins, J . Optical Sot, Am., 37,279 (1947).
’
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Mar., 1953
tained as oxides and carbonates from Lindsay Light and Chemical C O . ~
Results It has been found that scandium may be separated from
many of the elements frequently associated with i t ,by OXtraction from a hydrochloric acid medium. Pertinent tracer data are given in Table I.
TABLE I EXTRACTION OF TRACER^ SCANDIUM, YTTRIUMA N D PROMETHIUM INTO T B P FROM AQUEOUS HYDROCHLORIC ACID K for aq. HCI of indicated conon.
3.0 M
Element
6.4 M
8.0M
32 50 sc 0.04 0,001 0.05 Y < ,001 3000; Gd, 2000; Tb, 900; Dy, 600; Ho, 40; Er, 10; Tm, 2; and Lu, 0.7. Neodymium was obtained in 90% yield with the following approximate decontaminations from various rare earths; Sm, 2; Eu, 3; Gd, 4 ; Tb, 7 ; Dy, 15; and Yb, 200. This series of experiments was extended, using feeds of various relative rare earth compositions, in order to establish that increasing extractability parallels increasing atomic number for all of the lanthanides. Consistently, Y fractionated between ,Dy and Ho, extractin'g only slightly less well than the latter. No inversions were found in a study involving HCl of concentrations ranging from 6 to 12 M . Since one very important disadvantage associated with the system described is the necessity for using relatively dilute solutions of rare earths owing to the limited solubiljty of their chlorides in concentrated hydrochloric acid, an investigation of nitric acid-TBP systems was undertaken. The data of a preliminary survey are listed in Table 111.
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TBP and was followed by six 25-ml. ortions of aqueous scrub 10 N i n NH4N03 and 0.2 N in HN&. The scrubs were assayed for La, Y and Sc by cycling them through two more portions of TBP and analyzing these TBP phases. Analogously the solvent phases were analyzed for Al, Mg and Ca by cycling three more 50-ml. portions of ammonium nitrate scrub through them and assaying the composite of these aqueous phases. The results are shown in Table IV.
TABLE IV EXTRACTION OF MACRORARE EARTHS FROM COMMON CONTAMINANTS BY EXTRACTION INTO T B P FROM AMMONIUM NITRATE Added element in phase, % Phase La Gd Y So A1 Mg Ca -100 -100 Combinedscrubs 1 (15) T. Moeller and a. E. %ernerss Chem. Rave., 37,97 (1945).
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mental plot the value of R required, using the same system, to divide the rare earths at any predetermined point may z be obtained. It is apparent that for the system 12 M HN03-TBP the partitioning point Iis far to the left for large values of R and W far to the right for small values of R. L w Unduly large values of R may be avoided W J by use of a more concentrated HN03 W 0 phase, and inconveniently small values a 0 of R may be avoided by dilution of the 30 TBP. C’ The increasing sharpness of the cut 20 w 0 with increasing q“ is shown graphically in the theoretical curves of Fig. 9. If Y K a the ratio of the K’s for successive rare - 0 earths is a constant, then a definite in-1.0 -0.8 -0.6 -0.4 -0.2 crement in the abscissa of Fig. 9 correLOQlo ( R K I . sponds to the difference between adjaFig. 9.-Theoretical steady state extraction in a system of an odd numcent rare earths. Consequently, in prinber of contractors with center feed, G / R = 1.333. ciple, a vertical grid, with such a spacing, it follows that for constant K , 2, is related to CTBP placed on Fig. 9 so as to cross a theoretical curve as kt a point Corresponding to the extraction of one specific rare earth will also cut the same curve at dZp/d[log CTSP] = -f/a points corresponding to the extraction of all of the where 2, is the atomic number of the (‘partitioning other rare ear‘ths. In practice, this is very nearly element,” i e . , the element dividing approximately true in the region in which the experimental value equally into the two effluent phases. Therefore, of F p can be determined with sufficient accuracy t o in the fractionation of the rare earths in a given make such a determination valid, as may be shown system using nitric acid of a fixed concentration and by use of the data of Tables V and VI. .ConseTBP of various concentrations, a plot of the atomic quently, such a plot may be used to calculate the number of the partitioning element against the experimental K . values of certain specific rare logarithm of the per cent. concentration of TBP earths using a given combination of q, n, R and G should be a straight line of slope -flu. in order to predict the behavior of the same chemiFor the system 12 M HN03-TBP (with Gulf cal system using a different combination of q, solvent BT diluent) the respective approximate n,R and G. values off and u are 3 and 0.2. Consequently for By such a process of reasoning it is concluded this system the slope should approximate -15. that Gd. differing from La by an increment of In the experiments reported in Table V the atomic seven in ‘atomic ;umber, shouid have been sepanukbers of the partitioning elements are 61, 65, rated from La with a decontamination factor of 66 and 70. The plot of these numbers against the approximately lo8 in the experiment reported in logarithm of the per cent. concentration of T B P Table VI, R = 0.25, if the assumptions that K(z+I, is a straight line of slope -18. From such an = 1.59K.z and that the K values are independent of experimental plot the concentration of TBP concentration in the ranges considered are valid. required, using the same system, to divide the rare Since these assumptions are known to be approxiearths at any atomic number between 61 and 71 mated in fact, it seems safe to assume that the may be obtained. decontamination factor achieved was a t least lo6 . It is apparent that in order to place the point of -106 as compared with the demonstrated dedivision to the left of 61 some other variable must contamination factor of greater than 8. It may be changed. One of the simplest ways to accom- be noted that Gd was demonstrated to be deconplish such an end is to increase the value of R. taminated from Sm by a facto; of approximately 30 On the assumption that the ratio of the distribu- in the same experiment. tion ratios of successive rare earths in the 12 M By use of the continuous countercurrent extracHN03-TBP system is 1.6 the expression tion technique it seems likely, on the basis of the foregoing data, that the rare earths may be sepa(1.6)Az = Ri/Rg may be set up. Consequently, for a given system rated from non-rare earth impurities and fractionin which R is the only variable, the ratio of G to R ated into subgroups on an industrial scale. Large being constant, the plot of the atomic number of scale isolation of specific single rare earths in high the partitioning element against the logarithm of purity also appears economically feasible. Preliminary fractionation of the less abundant R should be a straight line of slope -5. rare earths preparatory to final purification by an In the experiments reported in Table VI the partitioning elements have atomic numbqrs of 61 ion-exchange technique is another attractive possiand 64. When plotted against the logari;thms of bility. For example (Table VII) a mass reduction the corresponding R values the points he on a of 90 for a Tm-Yb-Lu fraction has been obtained. straight line of slope -5. From such fin experi- Such 8 technique would permit the loading of a w
u)
b
MECHANISM OF COAGULATION OF LYOPHOBIC SOLS
Mar., 1953
larger quantity of the desired lanthanide on a given bed of exchanger. The results of several experiments demonstrate the feasibility of using a T B P solution of the rare earths as the feed. Such an approach is especially promising, since the TBP feed may be prepared by contacting a solution of rare earth nitrates with TBP under conditions such that common contaminates such as AI, Mg, Ca, Na, etc., are removed before fractionation of the rare earths is undertaken. An equally important effect is the removal, by use of this procedure, of interfering ions such as phosphate and sulfate which
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lower the K’s of the rare earths thereby shifting the position of the partitioning element to an atomic number higher than that calculated on the basis of data obtained using rare earths free from such ions. It is felt that the major points of interest of this study, from %hepoint of view of complex-forming tendencies of the lanthanides, are the remarkable to, K z for a given constancy of the ratio of K ( Z + ~ TBP-nitric acid system and the inversion of the order of increasing extractability noted a t approximately 3 M (in the aqueous phase) nitric acid. The explanation of the inversion is not known.
THE MECHANISM OF COAGULATION OF LYOPHOBIC SOLS AS REVEALED THROUGH INVESTIGATIONS OF SILVER HALIDE SOLS I N S T A T U NASCENDIl *
BY Bozo T E ~ A K
IN COLLABORATION
WITH
E. MATIJEVIC, K. SHULZ,M. MIRNIK,J. HERAK,V. B. VOUK, M. SLUNJSKI, AND T.PALMAR S. B A B I ~J., KRATOHVIL
Laboratory of Physical Chemislry, Faculty of Science, University of Zagreb, Zagreb, Yugoslavia Receiued April 16, 1968
The coagulation values of several neutral elect,rolytes were determined for systems of ‘silver halide sols in stalu nascendi with various concentrations of stabilizing ions. The coagulations were observed tyndallometrically. The following electrolytes were tested: lithium, sodium, potassium, rubidium, cesium, calcium, uranyl, aluminum, thorium, and strychnine nitrate (sometimes of sulfate) in the cases of negative sols, and sodium nitrate, sulfate, dichromate, phosphate, acetate, propionate, butyrate, valerate and citrate in the cases of positive sols. In some systems the effect of hydrogen ion concentration was also systematically followed. Furt,her, the coagulation values of potassium, barium, lanthanum and thorium nitrate in water-ethanol solutions were determined for negative sols of silver bromide. The development of some tipica1 systems was observed also in water solutions of gelatin. It was shown that the systems with gelatin may be used for di erentiation of the coagulation processes from the processes of growth of primary particles. In respect to the interpretation of the results it was pointed out that there is nearly linear relationship between the logarithm of the coagulation value and Bjerrum’s critical distance for formation of stabilizing-coagulating ion-pairs. The similar relationship may be applied to systems with a changed dielectric constant (water-ethanol media).
In the process of precipitation of salts of small. solubility from a solution of reacting electrolytes three stages may be distinguished: (1) nucleation, (2) regular or irregular growth, and (3) coagulation. All three stages are very dependent on the concentrational and other conditions of the precipitating system. By a systematical variation of the conditions, it may be possible to guide the precipitation in such a way that the stages (1) and (2) are very little affected thus allowing the examination of the practically isolated stage (3). Actually, in a number of investigations we have been able to use the formation of heteropolar precipitates, especially, the precipitation of silver halides, as a very sensitive indicator for the coagulating effects of the neutral electrolytes.2 The coagulation of such sols in slatu nascendi should not be necessarily more complex than that of usual sols, while the simplicity of preparation of such systems, the definite ionic character of the colloidal (1) Presented at the International Congress of Pure and Applied Chemistry, New York, September, 1951. (2) B. Tetak, 2. physik. Chem., 191A,270 (1942); 192,101 (1943); Arhiv kem., 19, 19 (1947); B. Tetak and E. Matqevih, ibid., 19, 29 (1947): B. Tetak, E. Matijevih and K. Schulz, ibid., 20, 1 (1948); B. TeZak, ibid., 22, 26 (1950); J. Herak and B. Teiak, ibid., 22, 49 (1950); B. Tetak, E. MatijeviC and K. Sohulz, J. A m . Chem. Soc., 73, 1602, 1605 (1951); THISJOURNAL, 55, 1558, 1567 (1951).
particles, and the regularities of the phenomena observed, may be a valuable tool for clearing up not only the questions of the coagulating mechanism itself, but also its dependence on both controlling factors: that of the crystalline solid, and that of electrolytic solution, by which the composition of the critical transition layer on the surface of the precipitating particles, the so called methoric space,3is conditioned. In this respect me are presenting a glimpse of several series of experimental results which may be taken as representative for the work done in our laboratory. For the experimental details and the potentialities of the method and technique used, our recent communications4 may give the necessary information. The Typical Precipitation Curve.-The starting point should be to ascertain the stability and instability regions of the typical precipitation curves which may be obtained by taking the concentrations of the precipitation components reasonably small and constant, and varying systematically the excess (3) Wo. Ostwald, Kolloid-Z., 100, 2 (1942); B. Tetak, Arhiv hem.. 81, 93,96 (1949).
(4) B. Tekak, E. Matijevih and K. Sohulz, J . A m . Chem. Soc., 73, 1602, 1605 (1951): THISJOURNAL, 66, 1558, 1567 (1951). (5) B. Tetak, 2. physik. Cham., 1 7 M , 219 (1935).
